This invention relates generally to fuel cells, and specifically, to using measurements of the dielectric constant to control and measure critical operations of the fuel cell.
Fuel cells are becoming more widely used as an inexpensive, continuous source of energy for many different applications. Direct Methanol Fuel Cell Systems (DMFCs), in particular, have the potential to provide power for electronics at a significantly higher energy density than batteries, in a scalable, small system.
FIGS. 1 and 2 illustrate two variations of a basic fuel cell 1, well known in the art, which in this illustration, could be either a reformer based fuel cell as in FIG. 1, or a direct oxidation fuel cell as in FIG. 2. Whether reformer-based or direct oxidation, a fuel cell 1 comprises an anode chamber 11, an anode electrode 12, an electrolyte 13, a cathode electrode 14, and a cathode chamber 15. The two electrodes, namely anode electrode 12 and cathode electrode 14 are sandwiched around electrolyte 13 as shown.
In reformer-based fuel cells, hydrogen 16, extracted from a hydrocarbon source fuel such as, but not limited to, natural gas, methanol, ethanol, butane, propane, or even gasoline, is fed into anode chamber 11, while oxygen, or a gas comprising oxygen 17, such as air, is fed (or allowed to enter) to cathode chamber 15. Anode electrode 12 contains a catalyst that promotes the chemical reaction that causes hydrogen atoms from a source fuel to split into protons and electrons. The protons pass through electrolyte 13, but the electrons, which are unable to pass through electrolyte 13, must instead take a different path around electrolyte 13. This creates a current 19 which is utilized to provide power, before the electrons return to cathode electrode 14. With the aid of cathode electrode 14, the hydrogen protons and oxygen are reunited on the cathode side of the fuel cell to create the fuel cell waste product, namely, water 18. Because fuel cell 1 relies on electrochemical oxidation and not combustion, emissions are not nearly as significant as emissions from even the cleanest fuel combustion processes.
In a reformer-based fuel cell system such as is illustrated in FIG. 1, hydrocarbons with multiple carbon atoms can be used as fuel. These fuels generally have higher energy density than fuels used in direct oxidation systems, but must be reformed by a separate fuel reformer 10, to assist in extracting hydrogen from the source fuel. Reformation is a technically difficult process that consumes energy and increases complexity of the overall fuel cell system. Because direct oxidation fuel cell systems generally use fuels that are molecularly simple, no reformer is needed to promote the reaction that releases the protons and electrons to generate electricity. As such, direct oxidation systems are simpler and can be fabricated in a smaller volume.
FIG. 2 illustrates a Direct Oxidation Fuel Cell, as would be present in a DMFC. Other than the absence of a fuel reformer 10, schematically, a Direct Oxidation Fuel Cell is identical to a Reformer Based Fuel such as was illustrated in FIG. 1, though it may be made from different materials to account for the different electrochemistry of the direct oxidation source fuels (including methanol), as opposed to a reformed source fuel. A Direct Oxidation Fuel Cell generates current in a fashion similar to reformer-based fuel cell systems, wherein anode electrode 12 promotes the desired oxidation of the fuel (methanol) and electrons flow, supporting a load. However, because the source fuel for a direct oxidation fuel cell is generally in a different phase than a reformed fuel, and because there are different by-products of the electricity-producing reaction, direct oxidation fuel cells have different ancillary support systems than reformer-based fuel cells.
In a DMFC, a liquid methanol (CH3OH) and water (H2O) mix 24 enters fuel cell 1 directly at anode chamber 11, as opposed to a reformer-based fuel cell wherein a reformed fuel containing hydrogen 16 (H2) extracted with the aid of the reformer 10 enters anode chamber 11. At anode electrode 12, methanol is oxidized according to:
CH3OH+H2Oxe2x86x92CO2+6H++6e.xe2x80x83xe2x80x83(1)
The CO2 23, is discharged as a waste product from anode chamber 11. The hydrogen ions (H+) pass through the membrane electrolyte 13, which may comprise, for example, Nafion(copyright), a commercially available material. Electrons (e) do not pass through the membrane electrolyte, and must take a different path through the load, creating a usable current 19. At cathode electrode 14, the oxygen 17 (O2) reunites with the electrons (e) from current 19 and the hydrogen ions (H+) according to:
O2+4e+4H+xe2x86x922H2O,xe2x80x83xe2x80x83(2)
thus creating water 18 (H2O) as a waste product. The overall chemical reaction of the DMFC 1 is therefore given by:
CH3OH+3/2O2+2H2O.xe2x80x83xe2x80x83(3)
DMFC 1 also comprises an anode gas diffusion layer (GDL) 21, and a cathode gas diffusion layer 22, which are utilized to ensure that the fluids involved in these reactions are diffused in a substantially uniform manner over anode chamber 11 and cathode chamber 15 respectively. The gas diffusion layers 21 and 22 are also typically part of a reformer-based system such as illustrated by FIG. 1, but are omitted from FIG. 1 for simplicity of illustration.
As noted in the earlier discussion and specified in eq. 1, pure methanol is not fed to anode chamber 11. Rather, to operate fuel cell 1 at peak efficiency, it is preferred to feed to anode chamber 11 a dilute mixture of methanol and water. In particular, it is well known in the art that the membrane electrolyte 13 is, to varying degrees permeable to water, methanol, and protons. As such, if the methanol concentration is too high relative to the water on the anode side of the DMFC, some methanol will pass through electrolyte 13 and react with the source of oxygen or air 17 without contributing to current 19. This reduces the efficiency of the DMFC, and wastes methanol. Alternatively, if not enough methanol is supplied, fuel cell 1 will not receive enough fuel to generate the desired current 19.
A 3% methanol, 97% water mixture is typical using current technology and load requirements. However, it is anticipated that over the longer term, this concentration might be as low as 2% or even 1%, but may become substantially higher as advances in the fuel cell 1, the electrolyte 13, and the ancillary systems are realized. As such, variations in methanol concentration are to be considered within the scope of this disclosure and its associated claims, and these may run as high as 5%, 10%, 15%, 30%, 50%, 75%, 90%, and even 100% as the fuel cell 1 and fuel cell system technology progresses.
More generally, the exact fuel and water mixing proportion in any given fuel cell application is related to the particular technology of fuel cell 1 and the overall fuel cell system which comprises fuel cell 1, and it is expected that these technologies will improve over time. Thus the desired mixing proportions will change as well. This anticipated change in optimum mixing proportions as these technologies progress is considered to be within the scope of this disclosure and its associated claims.
The water 18 produced as a by-product of DMFC operation is suitable as a water supply for mixing with the methanol source fuel, and indeed, is an attractive source for diluting water. In particular, provided that suitable methods for managing water are implemented, a DMFC system may be self-contained because the cathode-side fuel cell 1 reaction produces adequate water to operate the DMFC. It may, however, be necessary to remove some water from the DMFC to prevent saturation of the cathode electrode 14 of fuel cell 1. To ensure a proper mix of methanol and water, it is necessary to measure the relative concentration of methanol and water being supplied to anode chamber 11 on either a continuous or periodic basis. Further, it is necessary to utilize the measurements of the methanol and water mix as a basis for controlling the methanol and water concentration, in a feedback loop process.
Additionally, a fuel cell system often comprises a fuel tank (302 in FIG. 3) which stores the hydrocarbon source fuel such as methanol, whether in a pure form or pre-diluted with water. Of course, when the fuel is entirely consumed, no fuel will be delivered to the fuel cell, and the DMFC will no longer operate. As such it is desirable to measure the source fuel remaining in fuel tank 302, again on at least a periodic basis, so that additional source fuel can be supplied as needed.
While batteries and fuel cells each produce electricity through electrochemical reactions, there are fundamental differences that distinguish them from one another.
A battery is essentially a closed system whose effective cycle is limited, in part, by the amount the reactants that can be held within such system at any time. A battery generates electricity via a chemical reaction at the anode and the cathode with the electrolyte serving as the media for the transport of the reactants, creating a current that can be used to support an electric load. During this reaction, a battery""s anode, cathode, and electrolyte are reacted and consumed causing the battery to lose its ability to generate a current. A battery can be discharged only once before it must be replaced, or recharged by reversing the electrochemical reaction that causes the battery to discharge its energy. This reaction is generally reversed by passing a current through the battery, which also reverses the reaction that has consumed the electrodes and electrolyte. It is impossible to increase either the discharge time or maximum output of a particular battery without increasing the size of the battery proportionally.
Like a battery, a fuel cell generates an electrical current through a chemical reaction. However, unlike a battery, the electrodes and electrolyte in a fuel cell are not degraded or significantly altered by the reaction that generates electric current, making it possible to generate electricity for as long as source fuel is supplied to the fuel cell system. Because a fuel cell does not have to be a closed system, it is possible to add source fuel either periodically or continuously, allowing the system to generate electricity without interruption. In addition, the overall energy output of a fuel cell is related to the amount of source fuel supplied (which can be increased with time), rather than the size of the battery, making it possible for comparatively small fuel cells to deliver, over time, the same amount, or more energy, than a large battery.
It is, of course, well known that capacitive measurement of dielectric constants can be used to determine certain characteristics of substances for which this dielectric constant is measured. U.S. Pat. No. 4,438,182, for example, discloses a method for determining the life of a battery, by measuring the dielectric constant of the battery electrolyte over time. However, among other differences, this patent is entirely concerned with measuring the changes in the dielectric properties of a battery electrolyte to infer a state of charge, whereas in a direct methanol fuel cell, the characteristics of the membrane electrolyte do not change significantly, and are not measured. Rather, for a DMFC system, the concern is with directly measuring fuel concentration, and source fuel level, which are unrelated to the state of the electrolyte.
In addition, a battery has fixed amounts of reactants, which are not xe2x80x9cconsumedxe2x80x9d like a liquid fuel. Rather they react with one another and are chemically changed, and the volume of a battery""s reactants, in aggregate, remains constant as electricity is provided. Because of its construction, and the physical characteristics of its reactants, a battery typically can be oriented in any direction, without adverse impact on measurements that might be taken of the reactants. However, in a DMFC, as fuel is consumed, a fuel cell tank will comprise an ever-changing mix of air and hydrocarbon source fuel, such as methanol. If the Direct Oxidation Fuel Cell system, such as a DMFC, or indeed any fuel cell system, is reoriented, for example, turned on its side or upside down, methods of measuring the level of a source fuel that depend on a fixed or constant orientation will be inaccurate. It is therefor desirable to find an orientation-independent method to measure the source fuel level.
It is therefore desirable to provide a simple, low-cost apparatus and method to measure the relative quantities of water and methanol being mixed together and supplied to the anode of a DMFC, in order to optimize the efficient consumption of methanol source fuel by the DMFC.
It is further desirable to utilize these measurements of the relative quantities of water and methanol to control the subsequent mixing of water and methanol, in a feedback loop process.
It is further desirable to provide a simple, low-cost apparatus and method to monitor the source fuel remaining in the fuel tank of a DMFC.
It is further desirable for this apparatus and method to monitor the source fuel remaining in the fuel tank of a DMFC to do so in a way that is independent of the orientation of the fuel cell.
It is further desirable for control and logic components in the DMFC to act upon data related to the amount of fuel that remains, and to automatically order additional fuel via a telecommunications link when the fuel level falls below a predetermined point.
For a direct oxidation fuel cell system in which the source fuel is diluted with a diluting fluid prior to entering the fuel cell generally, and for a Direct Methanol Fuel Cell System (DMFC) in which the methanol source fuel is diluted with water, the dielectric constant of the fuel mix comprising the source fuel and the diluting fluid is measured to determine the relative proportions of source fuel and diluting fluid within this fuel mix. This measurement may then be used in a feedback loop to control the subsequent mixing of the source fuel with the diluting fluid, and in particular, to adjust the mix in the event the fuel mix is too rich or too dilute as compared to a desired mixing proportion. Additionally, a second dielectric constant measurement is used to determine the source fuel level of a fuel tank providing source fuel to the fuel cell.
In a preferred embodiment, a fuel cell system comprises a fuel cell mixing apparatus such as a chamber into which can be fed a source fuel and a diluting fluid, capable of mixing the source fuel and the diluting fluid together in a desired mixing proportion into a fuel mix, and capable of outputting the fuel mix for feeding to a fuel cell. This system further comprises a fuel mix dielectric constant sensor capable of measuring a fuel mix dielectric constant of the fuel mix output from the fuel cell mixing apparatus, thereby enabling the relative proportions of the source fuel and the diluting fluid within the fuel mix to be determined from the fuel mix dielectric constant. The measurement of the fuel mix dielectric constant is then used in a feedback loop as a basis to adjust, as needed, the mixing the source fuel and the diluting fluid together, in order to maintain the desired mixing proportion.